No heat in one zone: Need help bleeding system

Hi,

I have a forced air oil fired boiler with 3 zones (2 for heat and 1 for hot water) and 2 air handler units. Out of the 3 zones one is not working (2nd floor). I am not sure why one zone is not working.

I see in the boiler controller that the 2nd floor LED is red and I think it means that it is calling for heat. The pipes at the boiler for water return and water inlet to the 2nd floor are hot. The water inlet at the 2nd floor air handler is also hot. But I am not getting hot air from the vents. One of the things I heard is that this can be caused by air pockets in the line and I should try bleeding the system. I am not sure how it is done. Can anyone please guide me through it? How should I do this?